This invention relates to polymeric pigment dispersants, more particularly, it relates to graft copolymer pigment dispersants having an acetoacetyl amine pigment anchoring group. These dispersants are easy to prepare and are useful in dispersing a wide variety of pigments.
Polymeric pigment dispersants which are effective for dispersing pigments in organic liquids are known in the art and are used to form pigment dispersions that are used in a variety of solvent borne coating compositions. Nowadays, such pigment dispersions are widely used, for example, in exterior solvent borne paints for automobiles and trucks.
Much of the past activity with polymeric dispersants has been with random copolymers, but these relatively inefficient materials are being replaced by structured pigment dispersants having AB block copolymer or graft structures. The graft copolymer dispersants that have been used in the past are described in, for example, Huybrechts U.S. Pat. No. 5,852,123 issued Dec. 22, 1998. Such graft copolymers include a polymeric backbone and macromonomer side chains grafted onto the backbone and have attached to either the macromonomer or backbone, a polar group known as a pigment anchoring group which is designed to adsorb on the surface of a pigment particle and so attach the copolymer dispersant to the pigment surface. There is still a need to improve the binding or anchoring of these dispersants to the pigment surfaces. Ineffective anchoring of the dispersant to a pigment particle surface is highly undesired, as it allows the pigment particles to come close enough together to flocculate and leads to pigment dispersions and ultimately paints of poor stability and rheology and reduced color strength.
Nowadays, many of the modern pigments are chemically or physically treated to incorporate functional groups on their surfaces to enhance their performance. This presents the possibility for enhancing the binding force of a polymeric dispersant to the pigment surfaces, since these functional groups can then become potential sites for anchoring the dispersant polymers onto their surfaces for improved dispersion stability and rheology. The commonly used surface treating agents are pigment derivatives having acidic groups such as sulfonates and carboxylates. Naturally, a dispersant polymer with basic amino groups will be able to have a stronger binding force through the acid-base interaction with these acidic groups and become more effective.
There are several direct and indirect methods for introducing the basic amine functional groups into a dispersant polymer. Yet, they all suffer from certain significant drawbacks. For example, amine containing monomers can be directly copolymerized into the dispersant polymer during the synthesis. However, the commercially available amine containing monomers are few. The amine groups can also be introduced by reacting an amine compound with the epoxide groups that are built into a polymer through a monomer like glycidyl methacrylate. However, only the secondary amines can be cleanly reacted with the epoxide groups without crosslinking/gelling the polymers. The choice is also limited.
Therefore, there is still a need for new chemistries and convenient methods to broaden the choices of the types of amine groups in order to optimize the performance of the pigment dispersants described above.
The present invention provides a composition suitable for use as a pigment dispersant, which comprises a graft copolymer, preferably an acrylic graft copolymer, wherein the graft copolymer has a weight average molecular weight of about 3,000-100,000 and comprises about 10-90% by weight of a polymeric backbone and correspondingly about 90-10% by weight of macromonomer side chains attached to the backbone wherein
(1) the polymeric backbone is formed from polymerized ethylenically unsaturated monomers and
(2) the side chains are macromonomers that are attached to the backbone at a single terminal point and formed from polymerized ethylenically unsaturated monomers and have a weight average molecular weight of about 1,000-30,000;
wherein the graft copolymer contains about 2 to 70% by weight, based on the total weight of the graft copolymer, of polymerized ethylenically unsaturated monomers containing functional acetoacetate groups that are polymerized into the backbone, the side chains or both, wherein the acetoacetate groups of the copolymer are reacted with a compound bearing a primary amine group to form an acetoacetyl amine pigment anchoring group on the graft copolymer.
The present invention also provides stable and non-flocculating pigment dispersions formed by combining the pigment dispersant of this invention with any number of commercially available pigments and an appropriate organic liquid carrier. These dispersions are particularly useful in solvent borne coatings, especially automotive paints, where they impart uniform color to the paint and, at the same time, provide improved efficiency of pigment use, lower paint viscosity, and reduced emission of volatile organic solvents.
The novel pigment dispersant of this invention comprises a graft copolymer formed by the copolymerization of ethylenically unsaturated backbone monomers in the presence of a macromonomer. The macromonomer, which has only one terminal ethylenically unsaturated group, forms the side chains of the graft copolymer and is prepared first. It is then copolymerized with ethylenically unsaturated monomers chosen for the backbone composition to form the graft structure.
The graft copolymer contains about 10-90% by weight, preferably about 20-80% by weight, of polymeric backbone and correspondingly about 90-10% by weight, preferably about 80-20% by weight, of side chains. The graft copolymer has a weight average molecular weight of about 3,000-100,000 and preferably about 10,000-40,000. The side chains of the graft copolymer are formed from macromonomers that have a weight average molecular weight of about 1,000-30,000, and preferably about 2,000 to 15,000. All molecular weights referred herein are determined by GPC (gel permeation chromatography) using a polymethyl methacrylate standard.
The macromonomer useful in the present invention contains only one terminal ethylenically unsaturated group which is polymerized into the backbone of the graft copolymer. The preferred macromonomer is formed from polymerized acrylic monomers and in particular primarily from polymerized monomers of methacrylic acid, its esters, or mixtures of these monomers. Preferred monomers include methacrylic acid, alkyl methacrylates, cycloaliphatic methacrylates, and aryl methacrylates. Typical alkyl methacrylates that can be used have 1-18 carbon atoms in the alkyl group such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, pentyl methacrylate, hexyl methacrylate, 2-ethyl hexyl methacrylate, nonyl methacrylate, lauryl methacrylate, stearyl methacrylate, 2-(2-methoxyethoxy)ethyl methacrylate, ethoxytriethyleneglycol methacrylate, and the like. Cycloaliphatic methacrylates also can be used such as trimethylcyclohexyl methacrylate, t-butyl cyclohexyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate, and the like. Aryl methacrylates also can be used such as benzyl methacrylate, phenyl methacrylate, and the like.
Other ethylenically unsaturated monomers can also be used for forming the macromonomer such as acrylic acid, alkyl acrylates, cycloaliphatic acrylates, and aryl acrylates can be used. Preferred alkyl acrylates have 1-18 carbon atoms in the alkyl group such as methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, 2-ethyl hexyl acrylate, nonyl acrylate, lauryl acrylate, 2-(2-methoxyethoxy)ethyl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate, and the like. Cycloaliphatic acrylates can be used such as cyclohexylacrylate, trimethylcyclohexylacrylate, t-butyl cyclohexyl acrylate, and the like. Aryl acrylates such as benzyl acrylate, 2-phenoxyethyl acrylate, and the like can also be used. Apart from acrylic monomers, other polymerizable monomers that can be used for forming the macromonomer include vinyl aromatics such as styrene, t-butyl styrene and vinyl toluene, and the like. Methacrylonitrile and acrylonitrile monomers can also be used.
To ensure that the resulting macromonomer only has one terminal ethylenically unsaturated group which will polymerize with the backbone monomers to form the graft copolymer, the macromonomers are most conveniently prepared by a free radical polymerization method wherein ethylenically unsaturated monomers chosen for the macromonomer composition are polymerized in the presence of a catalytic cobalt chain transfer agent containing a Co+2 group, a Co+3 group, or both. The macromonomer polymerization is carried out in an organic solvent or solvent blend using conventional polymerization initiators. Typically in the first step of the process for preparing the macromonomer, the monomers are blend with an inert organic solvent and a cobalt chain transfer agent and heated usually to the reflux temperature of the reaction mixture. In subsequent steps additional monomers and cobalt chain transfer agent and conventional azo or peroxide type polymerization initiators are added and polymerization is continued at reflux until a macromonomer is formed of the desired molecular weight.
Preferred cobalt chain transfer agents are described in U.S. Pat. No. 4,680,352 to Janowicz et al and U.S. Pat. No. 4,722,984 to Janowicz, hereby incorporated by reference in their entirety. Most preferred cobalt chain transfer agents are pentacyano cobaltate (II), diaquabis (borondiflurodimethylglyoximato) cobaltate (II), and diaquabis (borondifluorophenylglyoximato) cobaltate (II). Typically these chain transfer agents are used at concentrations of about 2-5000 ppm based upon the particular monomers being polymerized and the desired molecular weight. By using such concentrations, macromonomers having the desired molecular weight can be conveniently prepared.
After the macromonomer is formed as described above, solvent is optionally stripped off and the backbone monomers are added to the macromonomer along with additional solvent and polymerization initiator, in order to prepare the basic graft copolymer structure by conventional free radical polymerization. The backbone monomers are copolymerized with the macromonomers via the single terminal unsaturated group of the macromonomer using any of the conventional azo or peroxide type initiators and organic solvents as described above. The backbone so formed contains polymerized ethylenically unsaturated monomers and any of the monomers listed above for use in the macromonomer may also be used in the backbone. Preferably, the backbone is formed from polymerized acrylic monomers, in particular primarily from polymerized acrylic acid, alkyl acrylate, cycloaliphatic acrylate, and aryl acrylate monomers as are listed above. Other preferred monomers include methacrylic acid, alkyl methacrylate, cycloaliphatic methacrylate, or aryl methacrylate monomers as are listed above. Polymerization is generally continued at the reflux temperature of the reaction mixture until a graft copolymer is formed having the desired molecular weight.
Typical solvents that can be used to form the macromonomer or the graft copolymer are alcohols, such as methanol, ethanol, n-propanol, and isopropanol; ketones, such as acetone, butanone, pentanone, hexanone, and methyl ethyl ketone; alkyl esters of acetic, propionic, and butyric acids, such as ethyl acetate, butyl acetate, and amyl acetate; ethers, such as tetrahydrofuran, diethyl ether, and ethylene glycol and polyethylene glycol monoalkyl and dialkyl ethers such as cellosolves and carbitols; and, glycols such as ethylene glycol and propylene glycol; and mixtures thereof.
Any of the commonly used azo or peroxy polymerization initiators can be used for preparation of the macromonomer or graft copolymer provided it has solubility in the solution of the solvents and the monomer mixture, and has an appropriate half life at the temperature of polymerization. xe2x80x9cAppropriate half lifexe2x80x9d as used herein is a half life of about 10 minutes to 4 hours. Most preferred are azo type initiators such as 2,2xe2x80x2-azobis (isobutyronitrile), 2,2xe2x80x2-azobis (2,4-dimethylvaleronitrile), 2,2xe2x80x2-azobis (methylbutyronitrile), and 1,1xe2x80x2-azobis (cyanocyclohexane). Examples of peroxy based initiators are benzoyl peroxide, lauroyl peroxide, t-butyl peroxypivalate, t-butyl peroctoate which may also be used provided they do not adversely react with the chain transfer agents under the reaction conditions for macromonomers.
The graft copolymer of this invention also contains a polar pigment anchoring group attached to either or both the backbone or macromonomer side chains. Preferably, the pigment anchoring group is concentrated on the backbone of the graft copolymer. The pigment anchoring group employed in this invention is an acetoacetyl amine group which can be, and preferably is, obtained by copolymerizing ethylenically unsaturated monomers containing functional acetoacetate groups into the backbone or side chains or both and subsequently reacting the acetoacetate groups built in either or both the backbone or side chains with a primary amine. The reaction product acetoacetyl amine will be a 1/1 molar equivalent adduct of an acetoacetate group with a primary amine group. The reaction conditions are preferably chosen so that 100% of the acetoacetate groups are reacted, or as close to 100% as can be reasonably achieved, leaving essentially no unreacted acetoacetate groups in the dispersant molecule. Typically after the graft copolymer described above is formed, primary amine and additional solvent are added to the polymer solution and the reaction is continued until all the acetoacetate groups are reacted and the acetoacetyl amine anchoring groups are formed. Another approach to the introduction of acetoacetyl amine groups into the graft copolymer is by reacting acetoacetate monomers with a primary amine and subsequently polymerizing this acetoacetyl amine monomer into the backbone, side chain, or both.
A preferred ethylenically unsaturated acetoacetate functional monomer that is useful for introduction of acetoacetate functionality into the graft copolymer is acetoacetoxyethyl methacrylate. Examples of other monomers that can be used to introduce acetoacetate functionality into the graft copolymer include acetoacetoxyethyl acrylate, acetoacetoxypropyl methacrylate, acetoacetoxypropyl acrylate, allyl acetoacetate, acetoacetoxybutyl methacrylate, acetoacetoxybutyl acrylate, and the like. In general, any polymerizable hydroxy functional monomer can be converted to the corresponding acetoacetate by reaction with diketene or other suitable acetoacetating agent. Alternatively, the hydroxyl groups may be selectively built onto the polymer, either on the backbone or in the arms, through the use of hydroxyl containing monomers. They are subsequently treated with acetoacetating agent such as t-butyl acetoacetate at elevated temperature and converted to the acetoacetate groups of the invention.
Examples of primary amines which are useful for forming the anchoring groups are aromatic amines, aliphatic amines, and primary amines containing heterocyclic groups. Aromatic amines that can be used include N-benzylamine, phenethylamine, 4-phenylbutylamine, 2,2-diphenylethylamine, and the like. Aliphatic amines can also be used such as propylamine, butylamine, aminoethanol, 2-amino-1-butanol, N,N-dimethylaminopropylamine, and the like. Primary amines containing heterocyclic groups can also be advantageously used because additional interactions between the heterocyclic groups and the pigment surfaces may further enhance the dispersion stability. The heterocyclic group can be a mono- or dinuclear five to seven member ring containing one or more nitrogen atoms as part of the ring and optionally an oxygen and/or sulfur atom. Useful examples include 4-(aminoethyl)morpholine, 2-(2-aminoethyl)-1-methyl pyrrolidine, 1-(2-aminoethyl)pyrrolidine, 2-(2-aminoethyl)pyridine, 1-(2-aminoethyl)piperazine, 1-(2-aminoethyl)piperidine, 1-(3-aminopropyl)imidazole, 4-(3-aminopropyl)morpholine, 1-(3-aminopropyl)-2-pipecoline, 1-(3-aminopropyl)-2-pyrrolidinone, and the like. Primary amines containing heterocyclic imidazole groups are particularly preferred.
In certain embodiments, the primary amine compound may contain both primary amine functionality, for acetoacetyl amine formation, and tertiary amine functionality. In this case, the tertiary amine functional graft copolymer can be, and preferably is, treated with a proton source or an alkylating agent to form a cationic quaternary ammonium group on the graft copolymer as the pigment anchoring group. Total alkylation should be at least about 30% of the tertiary amine moieties, preferably at least about 50% up to about 100%. Typical alkylation agents include aralkyl halides, alkyl halides, alkyl toluene sulfonate, or trialkyl phosphates halides. Alkylation agents which have been found to be particularly satisfactory include, benzyl chloride, methyl toluene sulfonate, and dimethyl sulfate.
The amount of acetoacetate functional monomer required will vary from case to case depending upon the desired degree of pigment anchoring necessary for the particular end use application. Generally, the concentration of acetoacetate functional monomers that are used to form the pigment anchoring groups in the graft copolymer should be at least about 1% by weight, based on the total weight of the graft copolymer, to impart appropriate pigment anchoring functionality to the graft copolymer. At lower concentrations, there may not be sufficient interaction with the pigment to avoid flocculation, particularly in more polar solvents. The preferred concentration of these monomers is about 2 to about 70% by weight, and more preferably about 5-20% by weight, based on the total weight of the graft copolymer.
In addition to the acetoacetyl amine pigment anchoring groups, the graft copolymer may also contain one or more additional anchoring groups in the selected anchoring segment. Particularly useful anchoring groups that work nicely in conjunction with acetoacetyl amine anchoring groups, are acyclic or cyclic amide groups. These anchoring groups can be, and preferably are, obtained by copolymerizing ethylenically unsaturated monomers containing acyclic or cyclic amide functionality into the desired segment during its polymerization. Acrylic, methacrylic and other vinyl amide monomers are generally preferred.
Useful examples of monomers that can be used to introduce acyclic amide groups include methacrylamides such as N-methylmethacrylamide, N-ethylmethacrylamide, N-octylmethacrylamide, N-dodecylmethacrylamide, N-(isobutoxymethyl)methacrylamide, N-phenylmethacrylamide, N-benzyl methacrylamide, N,N-dimethyl methacrylamide, and the like and acrylamides such as N-methylacrylamide, N-ethylacrylamide, N-t-butylacrylamide, N-(isobutoxymethyl)acrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N,N-dibutylacrylamide, and the like. Other monomers that can be used to introduce cyclic amide groups include methacrylic and acrylic and other vinyl monomers bearing cyclic amide groups, especially N-vinyl-2-pyrrolidinone and the like. Generally, the graft copolymers may contain up to 20% by weight, based on the total weight of the copolymer, of such amide functional monomers.
In addition to the anchoring groups described above, the graft copolymer may also, and preferably does, contain up to about 30% by weight, based on the total weight of the graft copolymer, of ethylenically unsaturated monomers that contain functional groups, such as hydroxyl groups, that will react with the film forming components present in the paint composition which in turn enables the dispersant to become a permanent part of the final network. This structure enhances film adhesion, improves the overall mechanical properties of the paint in general, and prevents deterioration or delamination of the film upon aging, as may occur if the dispersant remained an unreacted component. The hydroxyl groups, for example, may be placed in the backbone or in the macromonomer arms, or both. The preferred location is in the segment with the pigment anchoring groups.
While a wide variety of ethylenically unsaturated monomers can be used to introduce hydroxyl groups into the desired segment during its polymerization, acrylic monomers and in particular hydroxy functional acrylate and methacrylate monomers are preferred. Hydroxy functional methacrylates that can be used include 2-hydroxyethyl methacrylate, 3-hydroxypropyl methacrylate, 4-hydroxylbutyl methacrylate, and the like. Hydroxyl acrylates can also be used such as 2-hydroxyethyl acrylate, 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, and the like.
Particularly useful graft copolymers of this invention are exemplified in the examples given below.
While not wishing to be bound by any particular theory, these graft polymers when used as pigment dispersants are thought to work by anchoring onto and forming a layer of polymer surrounding the pigment particle, which layer extends into the surrounding solvent medium to provide steric stabilization of the pigment particles. The pigment particles then do not come close enough to one another to flocculate, unless there is insufficient interaction between the dispersant polymer and the pigment surfaces. The pigment anchoring groups employed herein have been found to effectively interact with a much wider range of pigments, which enables the graft copolymers of the present invention to be selectively adsorbed by a wider range of pigments and not be displaced from pigment surfaces by polar solvents or other polar functional groups present in the paint system which could compete for adsorption on the pigment surfaces. Stable and non-flocculating dispersions or millbases can thus easily be formed from the graft copolymers of this invention.
To form a pigment dispersion or a millbase, pigments are typically added to the graft copolymer in the customary organic solvent or blend and are dispersed using conventional techniques such as high speed mixing, ball milling, sand grinding, attritor guiding, or two or three roll milling. The resulting pigment dispersion has a pigment to dispersant binder weight ratio of about 0.1/100 to 2000/100.
Any of the conventional pigments used in paints can be used to form the pigment dispersion. Examples of suitable pigments include metallic oxides such as titanium dioxide, iron oxides of various colors, and zinc oxide; carbon black; filler pigments such as talc, china clay, barytes, carbonates, and silicates; a wide variety of organic pigments such as quinacridones, phtalocyanines, perylenes, azo pigment, and indanthrones carbazoles such as carbazole violet, isoindolinones, isoindolons, thioindigio reds, and benzimidazolinones; and metallic flakes such as aluminum flake, pearlescent flakes, and the like.
It may be desirable to add other optical ingredients to the pigment dispersion such as antioxidants, flow control agents, UV stabilizers, light quenchers and absorbers, and rheology control agents such as fumed silica and microgels. Other film forming polymers can also be added such as acrylics, acrylourethanes, polyester urethanes, polyesters, alkyds, polyethers and the like.
Pigment dispersions of this invention can be added to a variety of solvent borne coating or paint compositions such as primers, primer surfacers, topcoats which may be monocoats, or basecoats of a clearcoat/basecoat finish. These compositions may contain film-forming polymers such as hydroxy functional acrylic and polyester resins and crosslinking agents such as blocked isocyanates, alkylated melamines, polyisocyanates, epoxy resins, and the like. Preferably, the graft copolymer contains functional groups that will become part of the final network structure by reacting with the crosslinkers.
The following examples illustrate the invention. All parts and percentages are on a weight basis unless otherwise indicated. All molecular weights are determined by (GPC) gel permeation chromatography using a polymethyl methacrylate standard. Mn represents number average molecular weight and Mw represents weight average molecular weight. All viscosity measurements are reported using a Gardner Holtz scale.